How do I save my rendered animation as a video/gif?
If a series of images has been rendered, you will find them in the folder where Blender saves its renders, which is C:\tmp by default (check the path under 'Output' in the Output Properties tab, shown below, to check what your current output folder is).
It could be you only rendered frame 120, as that's the current frame showing in your Render Result: maybe you hit 'Play' to check your animation, and started rendering using F12 (Render Image) once the animation got to frame 120.
If you go the Output Properties window in Blender, you'll find options, under 'Output', to save as file formats other than .PNG, such as .JPEG, .BMP, or video formats such as .AVI and .MPEG:

Be sure to set the 'Frame Start' and 'End' to the sequence of frames that contain your animation.
Use Ctrl+F12 to render an animation.
Just like you can convert the series of images it might have created into a video file, you can convert it to a .GIF file afterwards.